Electronic timing device



March 10, 1970 D. M. NEALE 3,500,051

ELECTRONIC TIMING DEVICE Filed latch 17. 1967 4 Sheets-Sheet 1 MRI FIG.|.

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United States Patent f US. Cl. 250-214 4 Claims ABSTRACT OF THE DISCLOSURE Electronic timing relays controlling exposure to light of photographic materials wherein linear adjustment of a variable resistance provides substantially logarithmic adjustment of the timed interval.

This invention relates to electronic timing relays and in particular to relays controlling exposure to light of photographic materials.

Electronic timing relays are known in which the time interval is made dependent upon the mains supply voltage, V in such a manner that compensation is provided over a useful range of mains supply voltage for the effect of variations in such supply voltage on the exposure integral of light against time of the photographic material. Thus, if the light intensity, L, provided by a lamp connected to said mains supply, varies in proportion to V then the timing relay is designed to provide a time interval proportional to V,,,-, the design of such timing relays is summarised in Cold Cathode Tube Circuit Design (Chapman and Hall Ltd., 1964, pp. 118- 125 by D. M. Neale.

In the above prior art, adjustment of exposure time is effected by adjustment of a variable resistance, R, through which a capacitor, C, is charged. The circuit arrangement provides a time interval directly proportional to the product CR. Now it is well known that adjustment of photographic exposure integral, E, is best made by logarithmic increments. If adjustment is made by turning a knob through an angle 0' the known type of circuit requires that But =Log E040 EuC.R 00: log R This means that, for the said known type of timing relay, the variable resistance R should approximate to a logarithmic resistance/rotation law. There are technical difficulties associated with the provision of such a variable resistance with total resistance value and resistance/rotation law each sufiiciently precisely determined to permit the use of a previously-prepared scale in co-operation with a knob on the rotative control shaft of the variable resistance.

The present invention provides an electronic timing relay in which linear adjustment of a variable resistance provides substantially logarithmic adjustment of the timed interval.

The present invention provides moreover that the timed interval shall be inversely proportional to the nth power of the supply voltage Where n has a positive finite value not exceeding 6.0.

According to the present invention, there is provided an electronic timing relay which comprises a source of unidirectional voltage across which are connected in series a first fixed resistance, a second fixed resistance and Cir 3,500,051 Patented Mar. 10, 1970 a variable third resistance, such that the current flowing through said third resistance is at all times equal to the current flowing through said first resistance, furthermore a capacitor connected on one pole to the junction of said first and second fixed resistances and charged through a fourth resistance, from the voltage developed across said second fixed resistance, relay means made operative when the sum of the voltages developed across the capacitor and the first resistance reaches a predetermined voltage value. v

In one particular form of the invention, the unidirectional voltage has a value substantially 1.8 times the said predetermined value, the first fixed resistance has a value substantially 0.78 times the value of the second fixed resistance, the maximum value of the variable resistance is substantially 1.25 times the value of the second fixed resistance and the fourth resistance is greater than 10 times the value of said second resistance.

In another form, the voltage source provides both unidirectional and sinusoidally alternating voltage components, the value of the unidirectional component being equal to half the peak-to-peak value of the alternating component, the unidirectional voltage component having moreover a value substantially 2.8 times said predetermined voltage value, the first fixed resistance has a value substantially 0.39 times the value of the second fixed resistance, the maximum value of the variable resistance is substantially 3.3 times the value of the second fixed resistance and the fourth resistance is greater than 10 times the value of said second resistance.

Apparatus according to the present invention is of particular use in controlling the exposure to light of photographic material. In this art it is known practice to use a photocell to measure the intensity of light to which the photographic material is to be exposed and thereby to determine the required setting of the timing relay. In particular, it is known practice to place a photo-conductive photocell in the path of printing light, to adjust a variable resistance until a Wheatstone bridge circuit is brought to a condition of balance, the photocell and variable resistance being connected as adjacent arms of said bridge circuit the variable resistance being used at its so-adjusted value to charge a capacitor in a timing circuit controlling the time of exposure of the photographic material to said printing light.

In the said particular known practice, the adjusted value of variable resistance is approximately inversely proportional to the intensity of light falling on the photocell. The known practice therefore suffers from the previously-rnentioned disadvantages, viz. that the variable resistance should preferably have a logarithmic resistance/ rotation law.

This disadvantage is minimised by use of the apparatus of the present invention and therefore in a preferred embodiment of the present invention, a photometer is provided in which substantially equal increments of a variable resistance R are required to operate a relay device when logarithmic incremental changes are made in the intensity of light falling on a photoconductive cell. In use, the photocell is placed in the path of printing light, the said variable resistance R is adjusted just to operate the relay device, and the so-adjusted variable resistance, or a sympathetically-adjusted resistance, is thereafter used as the variable third resistance, R in a timing relay circuit of the type specified above.

By sympathetically-adjusted it is meant that adjustment of the value of one of said variable resistances produces an equal or proportional change in the value of the other.

It is an advantage of the photometer circuit proposed here that further increments of resistance may be added or subtracted from said variable resistance in order to adjust the combination of photometer and timing relay to suit photogra hic materials of different sensitivity. Each such increment will produce substantially the same logarithmic change in sensitivity of the photometer circuit.

According to a further form of the invention, therefore, a variable resistance, R and a fixed resistance, R are connected in series across a source of voltage, the voltage developed across R is applied to a fixed resistance R and a photoconductive photocell, R connected in series, R is adjusted until the voltage developed across R is brought to a predetermined value, the so-adjusted value. of R or a sympathetically-adjusted resistance being used as the value of R in a timing relay circuit as described above.

According to a modified further form of the invention a variable resistance, R a fixed resistance, R and a further fixed resistance, R are connected in series across a source of voltage, the voltage developed across R is applied to a fixed resistance R and a photoconductive photocell, R connected in series, R is adjusted until the sum of the voltages developed across R and R is brought to a predetermined value, the so-adjusted value of R or a sympathetically-adjusted resistance being used as the value of R in a timing relay circuit as described above.

In a particular embodiment of said modified further form of the invention the voltage source provides both continuous and sinusoidally alternating voltage components, the value of the continuous component being equal to half the peak-to-peak value of the alternating component, the continuous voltage component having moreover a value substantially 2.8 times said pre determined value, the resistance R has a value substantially 0.22 times the value of resistance R and the variable resistance R has a maximum value substantially 5.4 times the value of R The following drawings will serve to illustrate the invention by way of example only, in which:

FIGURE 1 is a circuit diagram of a relay circuit according to the invention.

FIGURE 2 is a graph showing that the timed interval varies in a substantially logarithmic relation to the varied resistance up to a certain value of the variable resistance.

FIGURE 3 is a circuit diagram of another relay circuit according to the invention.

FIGURE 4 is a simple photometer circuit diagram.

FIGURE 5 is a graph showing the substantial logarithmic relation between R and the light intensity on R FIGURE 6 is a modified photometer circuit diagram.

FIGURE 7 is a circuit diagram of a combined photometer and timing relay according to the invention,

In FIG. 1, rectified current from the alternating current mains supply flows through rectifier MR1 to charge C to a voltage V Resistances R R and R are connected in series across the supply voltage V The voltage developed across R causes current to flow through resistance R charging capacitor C until the sum of the voltages across R and C equals the critical trigger-to cathode voltage V of the cold cathode trigger tube V It is well known that the time required for the voltage sum to reach this value is proportional to the product of the values of resistance R;- and capacitance C It is also known that said time is substantially proportional to V if the resistances values R and R are in the proportion given by:

(Equation 1) where e=2.718 approx.

It has now been found that when R in FIG. 1 is varied, the said time bears, over a useful range, a logarithmic relation to the value of R in circuit.

If, therefore, a timing circuit is required in which the timed interval is proportional to V R is made equal to O.78R Curve A in FIG. 2 shows that, with such a relation between R and R and with V equal to 1.8V the timed interval 1 varies in substantially logarithmic relation to R up to a value of R equal to 1.25R2.

FIG. 3 shows an alternative embodiment in which the positions of C and MR1 have been interchanged. In the arrangement shown in FIG. 3 the voltage V varies with time in a sinusoidal manner though remaining al ways of the same polarity. The voltage V therefore comprises two components: one a unidirectional voltage and the other an alternating voltage. The presence of an alternating voltage component in the voltage, V requires that in order to provide a timed interval proportional to V the ratio R /R must depart from the value given by Equation 1 above.

Thus if n=3.7, the ratio R /R should be approximately 0.39 when the circuit configuration of FIG. 3 is used. With a 240 v. R.M.S. alternating voltage mains input to the circuit of FIG. 3, the continuous voltage component V is 240 =349 v. Using a trigger tube for which V =123 v., a dependance of timed interval and the value of R in circuit for values of R lying between 1.5R and 3.3R

FIG. 4 represents a simple photometer circuit. Across the series combination of R and R a voltage V produced in the circuit shown in FIG. 3. R is a photoconductive photocell which is placed in the path of light to be assessed. R is a comparison resistance of the same order of magnitude as the resistance R when illuminated. R is chosen to be at least an order of magnitude smaller in resistance than R R is adjusted until the peak voltage developed across R is equal to the critical trigger-to-cathode breakdown voltage, V of trigger tube V R and R are chosen to restrict the anode current of V to a small value and are proportioned so that at the peak value of V the anode-to-cathode breakdown voltage of V is not exceeded.

Curve C in FIG. 5 shows that for values of R up to 4.0R the relation between the adjusted value of R and light intensity on R is substantially logarithmic. Curve C relates to an alternating voltage mains supply of 240 v. R.M.S. and V =l23 v.

FIG. 6 represents a modified form of photometer circuit in which the additional resistance, R provides a reduction in voltage applied to the photocell, R Curve D in FIG. 5 shows that for values of R lying between 1.0R and 5.0R the relation between adjusted value of R and light intensity is substantially logarithmic. Curve D relates to a value of R equal to 0.224R an alternating mains supply voltage of 240 v. R.M.S. and V =l23 v.

FIG. 7 represents the circuit of a combined photometer and timing relay according to the invention. Switches SWla, SWlb, and SW10 are coupled. When in the position T as drawn, the circuit functions as a timing relay of the type shown in FIG. 3. When the switches are moved to the opposite position (marked P), the circuit functions as a photometer of the type shown in FIG. 6.

Switch SW2 is coupled either to switch SW3 or to switch SW4. In the following description it will be assumed that SW2 is coupled to SW3 and that SW4 is independently adjustable. Clockwise rotation of SW2 successively removes R and R from circuit. Clockwise rotation of SW4 successively removes R and R from circuit. Clockwise rotation of SW3 successively removes C and C from circuit.

Terminals 1 and 2 are connected to an alternating voltage mains supply. Rectifier MR1 causes a unidirectional voltage to appear across capacitor C When switch SWla is moved from position P to position T, therefore, a current flows through relay coil RLA/3 to charge capacitor C Thus current is sufficient to energise relay RLA/3, operating contacts a/l, a/2 and a/3. Closure of contacts a/ 2 allows relay coil RLA/ 3 to remain energised by current flowing through resistors R and R Closure of contacts a/1 allows the timing capacitance, C to be discharged through resistance R To initiate a timed interval, the normally-closed pushbutton switch PB is briefly opened. This interrupts the path of current through R Relay coil RLA/3 thus becomes de-energised and relay contacts a/ 1, 11/2 and a/3 return to the positions indicated. Contacts a/3 connect the exposing lamp LE to the mains supply. Contacts a/1 open to allow the timing capacitance, C to be charged through the timing resistance, R from the continuous component of the voltage appearing across R The timing capacitance, C is represented by those capaciators C C C placed in circuit by switch SW3. The timing resistance, R is represented by those resistances, R R R placed in circuit by switch SW4.

When C has charged sufficiently, the sum of the voltages appearing across R and C will be sufiicient to raise the trigger potential of trigger tube V to the critical voltage V Tube V then conducts to energise relay coil RLA/3, operating contacts a/l, 11/2 and a/3 and thereby terminating the timed interval. Contacts a/3 operate to remove the mains supply from the exposing lamp, LE, and to energise safelight lamp, LS, contacts a/l discharge C Contacts a/ 2 close to provide a hold on circuit for relay RLA/3 through R When R is thus placed in shunt with the anode-to-cathode gape of V the anode potential of V falls below the value requiredto maintain a discharge. The discharge in V is thus extinguished.

The duration of the timed interval is proportional to the value of C selected by SW3, and to the value of R selected by SW4. It varies logarithmically also with the proportion of resistance RV1 placed in circuit. Furthermore the time interval varies inversely as the nth power of the mains supply voltage where n is approximately 3.7.

The sum of the resistances R R R left in circuit by switch SW2 represents the resistance referred to as R in connection with FIGS. 4 and 6.

In initially adjusting the apparatus, the coupled switches SW2 and SW3 are set to their mid positions, i.e. R R C and C are in circuit, R and C are out of circuit. Using any convenient negative, a range of test prints is made. For each test print a different timed exposure interval is given corresponding to a different combination of settings of SW4 and RV1. The setting producing a satisfactory print is noted and SW4 and RV1 are subsequently left at this setting unless a more sensitive or less sensitive photographic material is to be exposed or unless the exposing lamp, LE, is exchanged for one of different aetinic value.

When the correct settings of SW4 and RV1 have been thus determined, the photocell R is placed to receive light forming a critical part of the negative image, e.g. a picture highlight. Switches SWla, SWlb and SW10 are now set in position P and RV2 is adjusted to bring the photocell circuit to a balanced condition. As the control RV2 accommodates a range of light intensities equal only to 4 to 1, an optical attenuator (not shown) over the photocell may have to be adjusted to pass more or less light to the photocell.

The balanced condition of the photocell circuit is that in which the peak voltage appearing across R and R together is just equal to V the critical trigger-to-cathode potential of V When this is so, V will conduct. Current will then be drawn by the trigger electrode, so charging C in a sense which inhibits triggering on immediately succeeding cycles of the mains supply voltage. As the charge on C leaks away through R however, a condition is reached in which V will again conduct. Conduction of V is observable as a glow on the cathode. The balanced condition is thus observed as the condition in which the cathode glow appears and disappears at a low frequency e.g. once or twice per second. If the value of RV is too low the pulsations of cathode glow will increase in frequency. For very low values of RV the cathode glow may appear on every cycle of the mains supply voltage. If the value of RV is too high, the frequency of cathode glow pulsation will decrease in frequency and in the extreme condition no glow will occur.

Once the initial adjustment of the apparatus has been made as described above, the method of use is as follows:

Switches SWla, SW11), SW10 are first set at position P, so extinguishing the safelight lamp LS, and energising the exposing lamp LE. For each negative image to be printed, the photocell is placed to receive light forming the appropriate critical part of the negative image, e.g. the picture highlight. The settings of switch SW2 and variable resistance RV1 are next adjusted until the balanced condition is indicated by slow pulsation of the cathode glow of V Switches SWla, SWlb, SWlc are moved to position T, so extinguishing the exposing lamp LE and energising safelight lamp LS. Photocell R is removed from the negative image plane and the photographic material is placed in position for exposure. Pushbutton PB is briefly opened and a timed exposure interval is thereby initiated.

It will be appreciated that because it is coupled to switch SW2, switch SW3 provides step-Wise adjustment of C in proportion to the step-wise adjustment of R Moreover the circuit values are chosen so that the logarithmic relation of the timed interval to adjustment of RV1 is the same as the logarithmic relation of light intensity on R to adjusted value of RV1. Thus when SW2 and RV1 are adjusted to provide coarse and fine adjustment of the photocell circuit to provide a balanced condition in respect of a particular light intensity, adjustment of SW3 and RV1 provides the required adjustment of timed interval such that the timed interval is substantially inversely proportional to the measured light intensity.

In FIG. 7 the adjustable link 3, allows resistances R and R to be put in or out of circuit as required to accommodate the apparatus to alternative nominal mains supply voltages.

In FIG. 7, V is shown as a type of cold cathode trigger tube (Ericsson GPET) provided with a priming anode and shield anode. Resistance R restricts to a small current the discharge between priming anode and cathode, which discharge provides primary ionisation for the trigger-to-cathode discharge. The shield anode is held at a fraction of the main anode potential by resistances R and R The function of the shield anode is to inhibit breakdown between main anode and cathode when the latter two electrodes are required to support the voltage developed across C Resistance R limits the main anode voltage and current of V when the apparatus is used as a photometer. Resistance R restricts the current flowing through MR1 when the mains supply voltage is first applied.

I claim as my invention:

1. An apparatus for controlling photographic exposure which is characterized in that it comprises an electronic timing relay which comprises either a source of unidirectional voltage or a source which provides both unidirectional and sinusoidally alternating components across which source are connected in series a first fixed resistance, a second fixed resistance and a variable third resistance, such that the current flowing through said third resistance is at all times equal to the current flowing through said first resistance, furthermore a capacitor connected on one pole to the junction of said first and second fixed resistances and charged through a fourth resistance, from the voltage developed across said second fixed resistance, relay means made operative when the sum of the voltages developed across the capacitor and the first resistance reaches a predetermined voltage value and a photometer in circuit with said timing relay wherein a variable resistance R and a fixed resistance R are connected in series across a source of voltage, the voltage developed across R is applied to a fixed resistance R and a photoconductive photocell R connected in series, R is adjusted until the voltage developed across R is brought to a predetermined value, the so-adjusted value of R being used as the value of the variable third resistance of the electronic timing relay.

2. An apparatus for controlling photographic exposure which is characterized in that it comprises an electronic timing relay which comprises either a source of unidirectional voltage or a source which provides both unidirectional and sinusoidally alternating voltage components across which source are connected in series a first fixed resistance, a second fixed resistance and a variable third resistance, such that the current flowing through said third resistance is at all times equal to the current flowing through said first resistance, a capacitor connected on one pole to the junction of said first and second fixed resistances and charged through a fourth resistance, from the voltage developed across said second fixed resistance, relay means made operative when the sum of the voltages developed across the capacitor and the first resistance reaches a predetermined voltage value and a photometer in circuit with said timing relay wherein a variable resistance R and a fixed resistance R and a further fixed resistance R are connected in series across a source of voltage, the voltage developed across R is applied to a fixed resistance R and a photoconductive photocell R connected in series, R is adjusted until the sum of the voltages developed across R and R is brought to a predetermined value, the so-adjusted value of R being used as the value of the variable resistance of the electronic timing relay.

3. An apparatus for controlling photographic exposure which is characterized in that it comprises an electronic timing relay which comprises either a source of unidirectional voltage or a source which provides both unidirectional and sinusoidally alternating voltage components across which source are connected in series a first fixed resistance, a second fixed resistance and a variable third resistance, such that the current flowing through said third resistance is at all times equal to the current flowing through said first resistance, furthermore a capacitor connected on one pole to the junction of said first and! second fixed resistances and charged through a fourth resistance, from the voltage developed across said second fixed resistance, relay means made operative when the: sum of the voltages developed across the capacitor and the first resistance reaches a predetermined voltage value and a photometer in circuit with said timing relay wherein a voltage source provides both unidirectional and sinusoidally alternating voltage components, the value of the unidirectional component being equal to half the peak-topeak value of the alternating component, the unidirectional voltage component having moreover a value substantially 2.8 times said predetermined value, there being a variable resistance R and a fixed resistance R and a further fixed resistance R which are connected in series across the voltage source, the voltage developed across R is applied to a fixed resistance R and a photoconductive photocell R connected in series, R is adjusted until the sum of the voltage developed across R and R is brought to a predetermined value of R which is used as the value of the variable resistance of the electronic timing relay.

4. An apparatus according to claim 3 wherein in the photometer the resistance R has a value substantially 0.22 times the value of resistance R and the variable resistance R has a maximum value substantially 5.4 times the value of R References Cited UNITED STATES PATENTS 2,274,158 2/1942 Penther 250-214 X 2,579,764 12/1951 Schwennesen 250214 X 2,666,858 1/1954 Levine 250214 3,220,304 11/1965 Clapp 35538 WALTER STOLWEIN, Primary Examiner US. Cl. X.R. 355--83 

